The control unit connects to and synchronises with the rotating or stationary asynchronous machine (ASM or DAM).
The procedure of identification of the mechanical rotary speed or the synchronous rotary field frequency in an asynchronous machine operated without an encoder is referred to in the literature as the ‘capture mode’. An associated circuit which permits such connection while avoiding current and torque peaks bears the technical name ‘capture circuit’.
The capture circuit is used in applications in which, for example, due to major mass moments of inertia of the drive, it is not possible to exclude the converter being connected to a drive which is still spinning. Further fields of use are found in the area of fan technology where at the time of connection the current rotary speed of the drive is not known and also cannot be measured for economic reasons.
Connecting a converter to a rotating asynchronous machine results in a high current and a torque surge which should be avoided, in dependence on residual remanence, slip and impressed voltage.
Methods of starting asynchronous machines in a situation with a stationary or rotating is rotor are known from various patent literature. For example, DE 38 20 125 C2 presents such a method. The method presented is for starting an inverter-fed encoder-less asynchronous motor which permits capture of a rotating drive without a residual flux being present in the machine. The method is based on an equivalent-circuit diagram, held in rotor field co-ordinates, of the ASM, in which the slip frequency is reconstructed from a measurement of the terminal current. The synchronous rotary frequency for the control voltage is ascertained from the calculated or estimated slip frequency and the impressed stator frequency. The change in time of the torque-forming current component is in that case added as a dynamic correction value to the motor frequency. The method converges in the synchronous rotary frequency of the drive and in that way permits identification of the synchronous speed.
DE 195 32 477 A1 describes a method for starting an encoder-less asynchronous machine which functions both with and also without a residual flux in the ASM uses zeroing of the torque formed. In that method the stator current vector is measured and a stator flux vector generated by the voltage is estimated or determined. From those values the moment caused at the motor is calculated and regulated to zero by means of a regulator. The system experiences excitation by a current pulse, from which it follows that upon possible rotation of the machine a torque is formed, which can be regulated as desired by the regulating system.
An essential feature of the last two methods mentioned is that a torque which is regulated to zero with a regulating system is calculated from the impressed voltage and the measured current. As a departure from that approach, DE 195 03 658 A1 presents a method which uses the residual remanence of the rotor flux and the voltage induced in the stator winding. When no residual rotor flux is present the method can also be used by excitation being effected with a voltage pulse.
An estimated value for the mechanical rotary field frequency of the rotor is derived from the rotary field frequency of the induced voltage vector which is ascertained by way of differentiation of the angle of the voltage vector measured at different times.
EP 469 177 A1 describes a method which also determines the rotary speed of an asynchronous motor, utilising the remanence effect. Evaluation of the terminal voltage induced by the remanence of the rotor has to be effected in that case by an additional circuit arrangement for voltage measurement as during the measurement operation no voltage is impressed by the converter and the final stage is blocked.
The method of DE 35 43 941 A1 uses the residual remanence of the system in a similar way. On the basis of the measured induced voltage, a rectangular signal of the same phase and frequency is generated, which is used for identification of the mechanical rotary speed or the synchronous rotary field frequency.
DE 35 43 983 A1 presents a method which is suitable for connecting a converter to a non-excited rotary machine which is still rotating. That method involves a search method which involves passing through the rotary frequency range with a constant reference current IS. On the basis of the change in the voltage to be impressed for that purpose (dU/dt) the search speed is adapted and reduced for major voltage changes. The mechanical rotary speed is detected on the basis of the increase in the flux in the region of the synchronous operating point. A disadvantage with that search method is the time required for identification of the mechanical rotary speed and the impressed torque which occurs because of the magnetisation effect.
DE 199 19 752 C1 describes a method which unlike the foregoing methods dispenses with calculation of the torque, the rotor flux or the moment or magnetisation current. In addition that method does not require any rotor residual remanence. In this method the reference value for the stator current vector is determined directly from the stator flux vector, wherein the direction of the current vector is selected substantially parallel to the direction of the flux vector. The method permits the use of the current regulator structures which exist in many cases (field-oriented regulation) and can be incorporated at minimum complication and expenditure. It only presupposes an expansion for calculation of the stator flux vector, which is determined by means of the impressed voltage and the measured current.